1. Field of the Invention
The present invention relates to a power factor correction circuit for improving a power factor of an off-line switching power supply in order to comply with the requirements of Class A or Class D stipulated in harmonic current rules IEC-1000-3-2.
2. Description of Related Art
A typical off-line switching power supply is shown in FIG. 1. The supply comprises an AC/DC rectifier 1, and a DC/DC converter 2 in which an electrolytic capacitor C.sub.1 is connected as a filter for the bridge rectifier DB.sub.1. As such, the electrolytic capacitor C.sub.1 begins to charge only when the bridge rectifier DB.sub.1 is conducting if the input ac voltage V.sub.s1 is higher than the voltage of the electrolytic capacitor C.sub.1. Note that the input current 1.sub.pc is a pulsating current as shown in the graph of FIG. 2. The power factor of the input current of the conventional off-line switching power supply is significantly decreased (e.g., approximately 50%), and the total harmonics distortion (hereinafter referred as "THD") is even higher than 100% after the rectification performed by the AC/DC rectifier 1. As a result, the total harmonics is seriously distorted, the quality is poor, and, even worse, the precious energy is wasted.
Thus, many countries have promulgated a number of harmonic current rules (e.g., IEC-1000-3-2) which specify the current waveshape of the power supply for manufacturers to obey in order to improve the efficiency and quality of the power source being supplied.
As such, various designs of power factor correction circuits have been proposed by researchers in order to improve power factor of the conventional off-line switching power supply. These designs have been located in a search as follows;
1. Inductor Type Power Factor Correction Circuit:
As shown in FIG. 3, the prior art discloses a design in which a low frequency large winding L.sub.1 is in series between a bridge rectifier DB.sub.1 and a electrolytic capacitor C.sub.1 The winding L.sub.1 and the capacitor C.sub.1 form a low pass filter to rectify the input current of a DC/DC converter 2. Such design is similar to the ballast for correcting the power factor of a fluorescent lamp in functionality. However, the winding L.sub.1 has the drawbacks of being relatively large, having only a limited power factor improvement, and abnormal high temperature developed.
2. Active Type Power Factor Correction Circuit:
As shown in FIG. 4, the prior art discloses a design in which the AC/DC rectifier is redesigned to form a two-stage circuit with the DC/DC converter 2. Further, a complex control circuit 11 and a large switch element Q.sub.1 are added therein to improve the power factor. However, it is relatively complex in circuit design and will cause high manufacturing cost.
3. Dither Type Power Factor Correction Circuit of Single-stage Single-switch:
The prior art shown in FIG. 5 is simple in circuit design. However, the whole circuit is redesigned, and a number of deficiencies have been found in use as follows:
a) The ripple voltage V.sub.dc will rise to approximately 100% to 200% if the load suddenly drops significantly when the DC/DC converter 2 is operating in a continuous current mode. As such, a high-voltage electrolytic capacitor is required. PA1 b) The alternating current component of the ac source V.sub.s1 will be brought into the DC/DC converter 2 when the switch element Q.sub.1 conducts. As a result, the output of the DC/DC converter 2 will be adversely affected by the 120 Hz ac voltage input, resulting in the rise of the ripple voltage. PA1 c) The large winding L.sub.1 hardly improves the power factor when the DC/DC converter 2 is operating in the continuous current mode.
4. U.S. Pat. No. 5,301,095 to S. Teramoto is disclosed in FIG. 6. Teramoto's patent replaces the diode D.sub.2 of the dither type circuit shown in FIG. 5 with a small capacity capacitor C.sub.3 in order to improve the power factor. However, the deficiencies of b) and c) as stated above are not effectively eliminated.
5. U.S. Pat. No. 5,600,546 to Fu-Sheng Tsai is disclosed in FIG. 7. Tsai's patent adds another winding L.sub.3, which is in series with the diode D.sub.2, into the dither type circuit shown in FIG. 5. Such design will resolve the problem of the rise of ripple voltage V.sub.dc of the electrolytic capacitor C.sub.1, when the DC/DC converter 2 is operating in continuous current mode, by lowering the induction ratio Lp/L1 of the primary winding L.sub.p of the DC/DC converter 2 to the winding L.sub.1, or increasing the induction ratio L.sub.3 /L.sub.p of the winding L.sub.3 to the winding L.sub.p. However, the deficiencies of a) and c) as stated above are not effectively eliminated.
Thus, it is desirable to provide a power factor correction circuit in order to overcome the above drawbacks of prior art.